1. Field of the Invention
The present invention relates to a microwave assisted magnetic recording apparatus suitable to provide data storage with high recording density, high recording capacity and a high data transfer rate.
2. Description of the Related Art
As a recording system that may solve the problem of thermal fluctuation in a magnetic recording medium, there is proposed a so-called “microwave assisted magnetic recording system”. In the microwave assisted magnetic recording system, a high-frequency magnetic field near the resonance frequency of a magnetic recording medium, which is sufficiently higher than a recording signal frequency, is locally applied to the magnetic recording medium. As a result, the magnetic recording medium resonates, and the coercivity (Hc) of the magnetic recording medium to which the high-frequency magnetic field is applied becomes half or less of its original value. Therefore, magnetic recording on a magnetic recording medium having a higher coercivity (Hc) and high anisotropy energy (Ku) is made possible by superimposing a high-frequency magnetic field on a recording magnetic field (e.g., U.S. Pat. No. 6,011,664).
In a method disclosed in the patent, however, a high-frequency magnetic field is produced with a coil. It has therefore been difficult to efficiently apply a high-frequency magnetic field during recording with high density.
To address this difficulty, there is also proposed a method of utilizing a spin torque oscillator as a device to produce a high-frequency magnetic field (e.g., U.S. Patent Application Publication Nos. 2005/0023938 and 2005/0219771). The spin torque oscillator as disclosed in these publications includes a spin injection layer, a non-magnetic layer, and a magnetic layer (hereinafter referred to as an “oscillation layer”). When a direct current (DC) is passed through the spin torque oscillator by means of an electrode, a spin torque produced by a spin injection layer causes ferromagnetic resonance in the magnetization of the oscillation layer. As a result, a high-frequency magnetic field is produced from the spin torque oscillator. Since the size of the spin torque oscillator is about several tens of nanometers, the produced high-frequency magnetic field is localized within about several tens of nanometers of the spin torque oscillator. Furthermore, the in-plane component of the high-frequency magnetic field allows a perpendicularly magnetized magnetic recording medium to efficiently resonate to significantly decrease the coercivity of the magnetic recording medium. As a result, high-density magnetic recording is performed only in a portion where a high-frequency magnetic field by the spin torque oscillator is superimposed on a recording magnetic field by the magnetic pole. This enables to make a magnetic recording medium having a high coercivity (Hc) and high anisotropy energy (Ku). Thus, it is possible to avoid the problem of thermal fluctuation during high-density recording.
However, even with the use of the spin torque oscillator, the intensity of the high-frequency magnetic field significantly decreases if the distance from the surface of the medium to the oscillation layer is increased. Therefore, there is a problem that achievement of microwave assisted magnetic recording is difficult.